1. Field of the Invention
The present invention relates to a method of ionizing a gas, and more particularly to a method of ionizing a gas within a cathode-containing chamber in which ionization of the gas is caused by the collision of thermoelectrons emitted by the cathode.
2. Description of the Prior Art
An ion implantor such as a Freeman type hot cathode ion source is shown in FIG. 7, to which reference should now be made.
A hot cathode 1 is heated by an electric current in the range of tens to hundreds amperes flowing through it to emit hot thermoelectrons. An electric voltage E1 in the range of tens to hundreds volts is applied between the hot cathode 1 and an ion chamber 4. Under these conditions a gas conduit 3 introduces a gas for ionization such as boron trifluoride, (BF.sub.3), phosphorus trifluoride (PF.sub.3) or arsenic as vapor. The gas collides with thermoelectrons and as a result is ionized to form a plasma in the ion chamber. A magnetic field is usually applied in parallel with the hot cathode 1 for increasing the mean free path of thermoelectrons and increasing the number of collision between the thermoelectrons and the supplied gas. This action results in increased an in the ion density and thus in a higher plasma density. Application of a of a voltage E2 between the ion chamber 4 and a withdrawing electrode 5 withdraws from the plasma through a slit through a slit 9 on the side of the ion chamber 4. In FIG. 7, the reference characters are designated as follows: 2 insulating material, 6 a power source of the hot cathode (filament), 7 a power source for producing arcs and 8 a power source for withdrawing ions.
In the ion source structured as mentioned above the cathode 1 is bombarded with cations in the plasma in the ion chamber 4 and, owing to the resulting sputtering effect, becomes thinner and thinner with time, and eventually the ion source becomes unusable. In an intermediate current ion implantor (this type is exemplified as ion source unless specified otherwise hereinafter), a tungsten hot cathode 1 is subjected to spattering, that results in becoming very thin as shown in FIG. 3 in several hours, and thus has only a short life of up to 100 hours, and requires a high frequency of replacements of cathodes.